Objectives: (1) Vertebrate cells contain sialic acid and sialyl transferase, while cultured mosquito cells (Aedes albopictus) lack both. Sindbis virus grown in vertebrate cells does, that from A. albopictus cells does not, contain sialic acid. Further work will delineate (a) possible biological and immunological properties related to this difference; (b) other biochemical differences; (2) The structural and biochemical changes induced in togavirus by complement to its potentiation of neutralization will be studied; (3) An agent inducing fusion of A. albopictus cells, possibly related to group B togaviruses and isolated from a "normal" A. aegypti cell line (Peleg), will be further characterized; (4) Defective-interfering particles and cell- associated RNA species involved in their generation in different cell types will be further characterized; (5) Dengue-2 virus will be grown in mosquito and vertebrate cells; comparative studies analogous to those done thus far with Sindbis virus will be initiated.